Photoresist removal (stripping) is a frequently used process in semiconductor integrated circuit (IC) fabrication. Photoresist is used to define particular patterns on wafers. It is used during lithography, ion implantation and plasma etching (where material other than the photoresist is removed), for example. After these processes, the photoresist is removed from the wafers before continuing to the next process.
Since photoresist stripping is used frequently in semiconductor manufacturing foundries, strippers are designed to have very short process time, i.e. high throughput, to reduce the overall wafer manufacturing cost. While different ways exist to increase a stripper's throughput, they fall into two categories: overhead reduction and strip rate improvement. Overhead includes wafer handling time, pump down time of the chamber into which the wafer is loaded, stabilization of pressure inside the chamber, wafer heating, and backfill of the chamber with a desired gas, all of which prepare a wafer for the particular process. The strip rate is a measure of how fast the photoresist is removed and cleaned from the wafer surface. The strip rate also determines how long a wafer is exposed to plasma. A wafer's exposure time to plasma in a strip chamber is generally minimized to reduce the possibility of electrical damage to various circuits on the wafer. The strip rate can be increased by using a higher plasma source power, higher wafer temperature, higher process gas flow or changing the gas chemistry.
Most strippers have an entrance hole through which a gas is injected into a chamber containing a wafer to be processed. The typical vertical distance between the entrance hole and the wafer is a few inches. This distance is minimized so that the chamber is compact and economical to manufacture. To obtain a uniform strip pattern, a uniform vertical flow for the gas at the wafer surface is maintained. At typical flow rates that are used, however, the gas will not fan out in a few inches. Thus, to achieve a uniform flow in such short distance, a gas dispersion system is used to disperse the gas stream to the wafer.
As shown in FIG. 1, a known stripper 100 contains a downstream chamber 102 in which the wafer 130 is exposed to the gas. The wafer 130 is held by a chuck 120. The gas 106 enters the downstream chamber 102 through an entrance hole 104. As the gas 106 enters the chamber, a gas dispersing system such as a baffle 110 disperses the gas 106 to distribute the gas 106 evenly onto the wafer 120. The strip uniformity and the strip rate are highly dependent upon this gas dispersing system. As shown in FIGS. 1 and 2, the baffle 110, 200 contains a large number of holes 112, 202 of different sizes. More specifically, the sizes of the holes increase with increasing distance from the center of the baffle because the center of the baffle receives more gas flow than does the edge. The gas 106, after acting on the wafer 120, exits from an exit port 108.
Other strippers 300 contain a downstream chamber 302 in which the wafer 330 is exposed to the gas as shown in FIG. 3. The wafer 330 is held by a chuck 320. The gas 306 enters the downstream chamber 302 through an entrance hole 304. As the gas 306 enters the chamber, a multiple baffle system baffle disperses the gas 306 to distribute the gas 306 evenly onto the wafer 320. The first baffle 310 contains holes 312, 314 of two different sizes similar to that described above. The second baffle 316 contains holes of only one size, which are offset from the holes in the first baffle 310 so gas molecules that pass through the holes on the first baffle 310 have to make two 90° turns before leaving the holes at the second baffle 316. The gas 306, after acting on the wafer 320, exits from an exit port 308.
Although not shown, in another design to disperse gas, a showerhead is used. A showerhead is similar to a baffle, however, the number and size of holes are such that they create a back pressure. Back pressures of about 10 Torr or greater are produced by such a design. The creation of these back pressures effectively slows down the gas flow above the showerhead and reduces the effect of flow dynamics.
However, it is complicated to optimize the hole sizes and pattern for the single baffle design. Baffles used in single baffle designs are also expensive to manufacture due to the various sizes and the large number of holes. Similarly, while multiple baffle designs may simplify the hole pattern, the use of multiple baffles increases the size and weight of the chamber, as well as increasing the cost of material, if not fabrication. In showerhead designs, the higher up stream pressure not only lowers the ionization efficiency of the gas source but also increases the radical recombination, and consequently lowers the strip rate.
Furthermore, the large surface area created by the baffles or showerhead and the internal shape of the upper chamber permit rapid neutralization of the radicals in the gas, which actually produce the stripping of the photoresist. Without a baffle, the stripping rate is two to three times as much as that with a baffle. This means that the baffle neutralizes more than half of the radicals generated by the gas source.